Internal or closed batch mixers have been commercially available for many years for mixing polymeric materials, such as plastics and rubber materials. The quality of the plastic or rubber products produced in the mixer is significantly influenced by mixing process parameters that are inherently variable, such as fluctuations in the mixer temperature, the cooling water temperature, the material feed temperature, the ram pressure, the rotor speed, the time of mixing, the fill level, the speed of the fill, and the like. These variations in the mixing process parameters influence the ejection temperature and mixing time, as well as the filler dispersion, viscosity, elasticity, homogeneity and percentage cure in the product. Poor mixing that leads to poor dispersion of filler particles, especially in rubber materials, can result in reduced product life, poor performance during service, poor product appearance, poor processing characteristics, and poor batch-to-batch product uniformity.
When batch mixers were introduced, control of the mixing operation to achieve satisfactory mixing was left almost entirely to the skill of the operator. However, instrumentation of batch mixers has been improved so that the information available to the operator to aid in control of mixing has increased and some automatic control has been introduced. For example, one proposed system would control the timing of addition of materials to the mixer, of raising of the plunger (ram) and of ejection of the load at predetermined "energy marks" based on a correlation between a superimposed desired or actual value control of the mixing temperature via the specific energy supplied to the mixer. Such a system proposes to eliminate differences in mixing qualities in the first charges of materials to the mixer on starting with a cold machine and during subsequent operation, as well as between summer and winter operation where there are significant differences in ambient factory temperatures.
Another proposed control system describes a process for mixing a polymer until it is estimated to attain a predetermined viscosity by measuring mixer rotor torque (corrected to a reference batch temperature) at a fixed rotor speed, determining the rate of change of torque, predicting the time to reach the predetermined viscosity by extrapolation, and thereafter continuing the mixing operation for the predicted time. A similar system measures the reaction torque experienced by the rotors and, in terms of a known relationship between torque and viscosity, assesses the relative Theological state of the compound in real time and adjusts the levels of kinetic and thermal energy inputs in order to achieve a predetermined value of viscosity on discharge. Problems exist, however, with each of these systems. For example, many compounds that are stiff or have high viscosity do not form a continuum within the mixer at the start of the mixing cycle and the initial process is inherently chaotic, with the result that the relationship between torque and viscosity cannot be defined with certainty. Moreover, the latter method involves the use of rotor torque as a measure of viscosity, viscosity as a measure of rheology, rheology as a measure of processability, and processability as a measure of extrudability. The relationship between rotor torque and extrudability is thus long and inherently inaccurate.
Many proposed systems attempt to control the whole mixing cycle automatically from the beginning. However, as described above, the problems in usefully controlling the mixing operation precisely during the initial, ingredient feeding stage are complex and these control systems are not very effective. For example, the various ingredients (in the case of rubber, carbon black, oil, fillers, curatives, antioxidants, etc.) are added to the mixer in a relatively short space of time which may be insufficient to operate the mixer to compensate for external variable factors, such as ambient factory temperatures, variation in feed stock, the quality of materials which, although nominally the same, are supplied by different manufacturers, and the temperature of the feed materials; and internal variable factors, such as the variable internal temperature of the mixer at the beginning of the mixing cycle after mixing of a previous batch, the cooling water temperature, the fill proportion, the speed of the fill, and the like.
Other proposed control systems attempt to control the mixing cycle only at a later stage, after all of the ingredients have been introduced to the mixing chamber and initially mixed. One such system monitors at least three mixing variables, such as the temperature of the mixed materials, the total power consumed, the time since the start of the mixing cycle, the torque applied to the rotors and the total number of revolutions of the rotors since the start of the mixing cycle. A series of complex equations relating these variables is then developed to produce target values at specified times during the mixing cycle for temperature and power, temperature and torque, temperature and rotor revolutions, rotor revolutions and torque, rotor revolutions and power, and torque and power. The system then changes the rotor speed and/or ram pressure at these specified times if the target values are not met. Because of the large number of different mixing parameters measured, this system is extremely complex for use during short mixing times after introduction of the materials, which for rubber processing are typically 1-3 minutes.
In view of the foregoing, there is still a need for a simple and efficient method for controlling parameters of the mixing process in order to produce polymer products having superior quality and uniformity from batch to batch.